Method for processing iron ore concentrates

ABSTRACT

A water slurry of an iron ore concentrate from concentration equipment, for example, magnetic separators in a primary circuit containing fine high grade mineral particles, coarse low grade middling particles and coarse high grade mineral particles, is pumped to a hydrocyclone in a regrind circuit. The water slurry is separated in the hydrocyclone into (1) an overflow fraction containing substantially all the fine high grade mineral particles and coarse low grade middling particles, and (2) an underflow fraction containing substantially all the coarse high grade mineral particles. The underflow fraction has a pulp density greater than 2.7 and preferably between 3.0 and 3.6. The overflow fraction is further classified into an undersize fraction containing substantially all the fine high grade mineral particles and an oversize fraction containing substantially all the coarse low grade middling particles. The oversize fraction is reground and reconcentrated prior to sending to subsequent treatment, such as pelletizing. The hydrocyclone underflow and the fine high grade mineral particles are sent to subsequent treatment, such as pelletizing, without regrinding.

United States Patent [191 Aubrey, Jr. 9

[ Feb 12,1974

[ METHOD FOR PROCESSING IRON ORE [73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

221 Filed: Dec. 20, 1972 21 Appl. No.: 316,963

[57] ABSTRACT A water slurry of an iron ore concentrate from concentration equipment, for example, magnetic separators in a primary circuit containing fine high grade mineral particles, coarse low grade middling particles and coarse high grade mineral particles, is pumped to a hydrocyclone in a regrind circuit. The water slurry is separated in the hydrocyclone into (1) an overflow fraction containing substantially all the fine high grade mineral particles and coarse low grade middling particles, and (2) an underflow fraction containing sub [52] US. Cl. 241/20, 241/24 Stantially a" the coarse gh grade i l particles [5 l Ill. Cl. B02C 25/00 The underflow fraction has apulp density greater than [58] Fleld of Search 241/16, 20,24, 68, 81 2 and p f y between d 3 The overflow fraction is further classified into an undersize fraction [56] References C'ted containing substantially all the fine high grade mineral UNITED STATES P TE T 'particles and an oversize fraction containing substan- 2,373,635 4 1945 Wuensch 241/24 x i lly all h coar low gra e middling particles. The 2,675,966 4/1954 Kihlstedt 241/20 oversize fraction is reground and reconcentrated prior 3,067,957 /19 E t 241/24 X to sending to subsequent treatment, such as pelletiz 3,337,328 8/I967 Lawver; 241/24 X The hydrocyclone d fl and the fi high L870 11/1968 Nfghlmgalew 241/24 X grade mineral particles are sent to subsequent treat- 3,502,27l 3/1970 Hays 241/20 ment such as penetizing, without regrinding Primary Examiner-William S. Lawson 5 Claims 2 Drawing Figures (l0. HO)

SLURRY ll FROM (u so) PRIMARY CIRCUIT (II Su) l IO HUI 2 PELLET I Z NG WASTE PAIENIED 3.791 .595

(IO HO) SLURRY FROM (n so) PRIMARY CIRCUIT (ll Su) PELLET 2| NG WASTE (I00 H0) (IO Ho) I (I00 Hu) PELLETIZI NG WASTE BACKGROUND OF THE INVENTION This invention is directed to an improved method for upgrading an iron ore concentrate wherein coarse high grade mineral particles are separated from fine high grade mineral particles and coarse low grade middling particles in a hydrocyclone classifier heavy media separator. I

It is conventional practice in the industry to crush and grind iron ores containing magnetite, hematite, and the like in a communication mill or mills in a closedcircuit with a classifier, such as a screw classifier or a hydrocyclone. In the comminution mill mineral particles are liberated from gangue particles as fine high grade mineral particles. (Actually the high grade mineral particles include a range of sizes including both fine particles and some coarse particles.) Not all of the mineral particles are liberated; some remain as coarse particles associated with gangue material as so called coarse low grade middlings. In the classifier, which accomplishes a size separation, the fine mineral particles are separated from the coarse particles prior to sending the ground material to concentration. Because of the difference in specific gravity between the particlesof gangue and mineral particles, some coarse ironcontaining low grade middling particles, which are relatively light in weight, overflow the classifier and are concentrated with the fine mineral particles. A minor amount of fine particles of gangue become separated also with the fine high grade mineral particles due to inefficiencies in the separation process, but the amount is small and unimportant.

It is often desirable or necessary to'further upgrade the primary iron ore concentrate. The general practice is to screen the iron ore concentrate from the separators and regrind and re'concentrate the entire coarse fraction of the iron ore concentrate in spite of the fact that it contains both fine and coarse high grade mineral particles which do not require further grinding. The coarse high grade mineral particles are naturally occurring particles coarser than the practical liberation size of the ore. The fine high grade mineral particles are formed during the crushing and grinding steps in the primary circuit and are smaller than the liberation size. Of course, it is well known that the actual practical liberation size varies for different ores. Associated with the coarse high grade and fine high grade particles are coarse middling particles which are composed of unliberated mineral particles associated with gangue material.

In the conventional regrind step, all the coarse particles are reground so that mineral values in the coarse low grade middling particles are liberated as high grade mineral particles. The regrinding, however, of naturally occurring coarse high grade mineral particles, which are present in the primary concentrate with the coarse low grade middling particles, results in unduly overgrinding the coarse high grade mineral particles and undesirably reduces the coarse high grade mineral particles in size. Charging the coarse high grade mineral particles into the regrind mill also increases the horsepower required to run the regrind mill and produces additional amounts of fines and slimes, which are difficult to treat in subsequent beneficiation steps and treatment such as thickening, filtering, and pelletizing. It is thus costly and inefficient to operate a closed-circuit grinding and beneficiating plant as in the above prior art practice.

It is the object of this invention to provide an improved method of separating coarse high grade mineral particles and fine high grade mineral particles from the coarse low grade middling particles in a water slurry of an iron ore concentrate from a primary circuit to prevent overgrinding of the coarse high grade mineral particles in a regrind and reconcentration operation.

It is another object of this invention to provide an improved method of separating coarse high grade mineral particles and fine high grade mineral particles from coarse low grade middling particles in a water slurry of an iron ore concentrate from a primary circuit prior to passing them to a regrind circuit to thereby reduce the load on the regrind and reconcentration equipment to reduce the horsepower consumption per ton of primary concentrate and to increase production.

It is another object of this invention to provide an improved method of separating coarse high grade mineral particles from coarse low grade middling particles in a water slurry of an iron ore concentrate from a primary circuit so that the material sent to subsequent treatment is coarser than heretofore attainable.

SUMMARY OF THE INVENTION Broadly, the improved method of the invention includes separating and recovering coarse high grade mineral particles from coarse low grade middling particles and fine high grade mineral particles contained in a water slurry of an iron ore concentrate from a primary circuit by treating the slurry of the iron ore concentrate in a hydrocyclone acting as a heavy media separator as well as a classifier. The fine high grade mineral particles are separated from the coarse low grade middling particles by subsequent classification prior to regrinding and reconcentration. The coarse and fine high grade mineral particles are sent to subsequent treatment without regrinding. The coarse low grade middling particles are sent to a regrind and reconcentration installation wherein high grade mineral values contained in the coarse low grade middling particles are liberated from the coarsegangue particles and recovered as usable fine high grade mineral particles.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of the improved method of the invention showing the steps of the method.

FIG. 2 is a variation of the improved method of the invention.

PREFERRED EMBODIMENT OF THE INVENTION A water slurry of aniron ore concentrate from primary concentration equipment, for example, magnetic separators, such as is produced in a closed-circuit crushing and grinding operation, contains coarse high grade mineral particles, coarse low grade middling particles, and relatively fine high grade mineral particles and can contain a minor portion of particles of gangue. The water slurry of the iron ore concentrate can, in accordance with the present invention, be treated to separate the coarse low grade middling particles from the coarse high grade mineral particles and fine high grade mineral particles prior to regrinding and reconcentration in a regrind and reconcentrating circuit. As a result, the capacity and throughput of the regrind mill in the regrind and reconcentration circuit is increased, and the overgrinding of the coarse high grade mineral particles is prevented. As the tonnage fed to the regrind mill is decreased, the power used to operate the regrind mill can be reduced or if it is desired to increase the capacity of the regrind mill, the power can remain constant.

As is well known, the presence of the coarse low grade middling particles in the water slurry of the iron ore concentrate has an undesirable effect on the grade of the concentrate. A screen analysis of a typical concentrate of the magnetite-type iron ore can contain the following mesh sizes and chemical compositions:

Mesh Size 72 lron in Silica in Concentrate Concentrate +100 22.3 28.l l +l50 38.5 19.2 l50 +200 60.0 6.5 200 +325 68.3 1.9 325 70.0 1.2

From the above, it can be seen that the practical liberation size of the ore is 200 mesh. All mesh sizes in this specification and claims are U.S.S. standard sieve size.

We have found that the water slurry of the iron ore concentrate of the above type, which contains coarse high grade mineral particles, coarse low grade middling particles, fine high grade mineral particles and a minor amount of particles of gangue, can be treated in a hydrocyclone to separate the coarse high grade mineral particles as an underflow and coarse low grade middling particles and fine high grade mineral particles as an overflow if the ore particles themselves are used as heavy media and the density of the underflow is controlled at a pulp density of not less than 2.7 and preferably between 3.0 and 3.6. The separation is made at 200 mesh particle size since, as noted above, there is a sharp drop in the content of middling, or low grade, particles at this size. Note that the particles of -200 mesh material contain at least 68 percent iron and about 2.0 percent silica. Since there is a sharp drop in the impurities in the particles below 200 mesh, the practical liberation size of this iron ore is 200 mesh. The fine high grade mineral particles, that is, particles of about 325-rnesh can be separated from coarse low grade middling particles by classifying, such as screening or in a second hydrocyclone or the like. The coarse .high grade mineral particles and fine high grade mineral particles are treated by physical ore treatment processes, such as filtration and the like, and can be sent to a pellet plant without further grinding. Of course, the coarse high grade mineral particles can be processed by means other than pelletizing, for example, briqu'etting, for use in metallurgical furnaces.

Turning now to FIG. 1 of the invention, the water slurry of the iron ore concentrate from primary concentration equipment (not shown) is pumped to a first hydrocyclone 10. We have shown one hydrocyclone, but it must be understood that a plurality of hydrocyclones can be used if increased capacity is desired.

The hydrocyclone is operated at a pressure drop sufficient to make a size separation on the coarse high grade mineral particles at a size equal to or smaller than the practical liberation size for the concentrate being treated. Of course, the liberation size will vary with the iron ore being treated. An underflow (10-HU) and an overflow (104-10) are formed in the hydrocyclone 10.

The pulp density of the underflow (IO-l-IU) is maintained at not less than about 2.7.and preferably between about 3.0 and about 3.6, which creates a heavy media effect thereby sending the coarse low grade middling particles, which are relatively light in weight, into the overflow (Ill-HO) along with fine high grade mineral particles. Under these special conditions, the underflow (10-HU) consists essentially of coarse high grade mineral particles, which are sent to subsequent processing, for example, pelletizing, without regrinding. The hydrocyclone overflow (IO-HO), because of the combination of heavy media and classification effects, contains coarse low grade middling particles, which contain unliberated high grade mineral particles associated with gangue, and already liberated fine high grade mineral particles. The overflow fraction (IO-HO) is classified by a classifying screen 11, which makes a separation equal to or finer than the practical liberation size. The screen 11 can be of the curved type generally referred to as a sieve bend. While we have shown only one curved screen, in the interests of increased production a plurality of screens can be used.

The overflow fraction (IO-HO) from the hydrocyclone 10 is split into two fractions on the screen 11. The fractions are a screen oversize (1lSO), that is, the particles which pass over the bars of the screen without passing through apertures between the .bars in the screen, and the screen undersize (1lSU) representing the particles which pass through the apertures between the bars of the screen. As noted in U. S. Pat. No. 2,916,142 issued Dec. 8, 1959 to F. N. Fontein the particles which pass through the screen are not more than one-half the size of the apertures between the bars. The efficiency of the screens can be increased by employingrapping as described in U. S. Pat. No. 3,446,349 issued May 27, 1969 to William Benzon. The screen overflow fraction (ll-SO) will contain substantially all of the coarse low grade middling particles since these particles are coarser than the practical liberation size. The screen undersize fraction (ll-SU) will contain substantially all the fine high grade'mineral particles and is sent without regrinding to subsequent treatment, such as pelletizing.

The screen overflow fraction (ll-SO) is sent to a regrind mill 12 and concentration equipment 13, for example, magnetic separators, in a closed regrind circuit.

wherein the fine high grade mineral values in the coarse low grade middling particles are liberated therefrom and are separated from the particles of gangue which are rejected and wasted. As is well known in the art, in a closed circuit the product of the grinding mill is conveyed to a classifier,..The material which is coarser than the desired maximum size'is automatically returned to the grinding circuit and only the fine fraction is advanced to the next step in the process. In this case the liberated fine high grade mineral particles are removed from the closed regrind circuit and recovered as a usable product and are sen't'to subsequent treatment. The particles of gangue are passed to waste.

The regrind mill 12 and reconcentrate circuit 13 can include a ball mill, a hydrocyclone and screen, as well as other apparatus well known in the art, such as flotation cells.

It must be understood that the regrind circuit can be operated in open circuit if so desired without changing the embodiment of the invention.

While the preferred embodiment has the hydrocyclone prior to the screen 11, as this decreases the tonnage to be screened and delivers a desirable feed to the screen, it is possible to operate the circuit with the screen 11 ahead of the hydrocyclone 10 without changing the scope of the invention.

Of course, ancillary equipment such as sumps, pumps, dewatering magnets, etc., are used as required in a plant in order to efficien'tly operate by the method of the invention.-

The hydrocyclone 10 and the screen 11 do not make perfect separations and some misplaced material will inevitably be present in the hydrocyclone overflow (IO-HO), hydrocyclone underflow (10-HU), screen overflow (ll-SO), and screen underflow (ll-SU) streams. Such misplaced material will be found to be minimal, however.

While we have described the invention with respect to iron ores and concentrates thereof, other high specific gravity minerals other than iron minerals, for example, zinc and lead minerals, are amendable to treatment by this invention.

Other types of classifiers can be used in place of screen 1 l, but the screen is preferred because of its relatively high efficiency.

FIG. 2 shows a variation of the method of the invention. We have used primed numbers to illustrate the variation shown in FIG. 2.

A classifying hydrocyclone 10a not operating under heavy media conditions is used in place of the screen 11 shown in FIG. 1 to separate the fine high grade mineral particles from the coarse low grade middling particles in the hydrocyclone overflow (IQ-HO). The fine high grade mineral particles form an overflow (IOa-HO) and the coarse low grade middling particles form an underflow (10a-HU). The overflow (l0al-IO) is sent to subsequent treatment, such as pelletizing, without regrinding while the underflow (l0a-HU) is passed to a regrind circuit which includes a regrind mill l2 and concentrating equipment 13.

In a specific example of the invention, a throughput of 280 tons per hour of a magnetite concentrate was processed in a beneficiating plant. The magnetite concentrate from the magnetic separators containing 3.4% SiO was mixed with water to form a slurry having a pulp density of 2.5. The slurry was passed to hydrocyclone operating under the desired conditions. The overflow fraction from the hydrocyclone was about 218 tons per hour and contained fine high grade mineral particles and coarse low grade middlings. The overflow fraction had a pulp density of 2.3. The hydrocyclone overflow fraction was screened on sieve bends. About 65 percent of the hydrocyclone overflow fraction was fine high grade mineral particles which passed through the screen and which were then passed to the pelletizing plant. The oversize screen fraction, constituting about 35 percent of the hydrocyclone oversize fraction, containing corase' low grade middling particles was recycled to a regrind ball mill for regrinding. The reground particles were processed in the regrind circuit to recover usable mineral particles, which were pumped to the pellet plant. The rejected gangue minerals were discarded.

The underflow fraction from the hydrocyclone, totaling about 62 tons per hour and having a pulp density of 3.1, was found to be substantially all coarse high grade mineral particles which analyzed 2.0 SiO All the underflow fraction was passed to the pelletizing plant without regrinding.

The combined material sent to the pellet plant contained 2.2% SiO compared to the primary concentrate analysis of 3.4%. Only 76 TPl-l were sent to the regrind mill, whereas in a conventional circuit app'ioi'ifiiiti TPl-l are sent to the regrind mill.

I claim:

1. An improved continuous method for separating and recovering mineral particles in a water slurry of a concentrate from a primary circuit, said water slurry concentrate consisting substantially of coarse high grade mineral particle's, coarse low grade middling particles, and fine high grade mineral particles, said method comprising:

a. treating said water slurry in a hydrocyclone operating under controlled conditions to form an underflow fraction containing substantially all the coarse high grade mineral particles and having a pulp density of greater than 2.7 and an overflow fraction constituting substantially of coarse low grade middling particles and fine high grade mineral particles,

b. passing the underflow fraction of step (a) to a subsequent treatment,

c. classifying the overflow fraction of step (a) into an oversize fraction consisting substantially of coarse low grade middling particles and an undersize fraction consisting substantially of fine high grade mineral particles, v i

d. passing the undersize fraction of step (c) consisting of fine high grade mineral particles to subsequent treatment,

e. passing the oversize fraction from step (c) to a regrind and reconcentration circuit wherein high grade mineral particles are liberated and recovered as a usable product and fine particles of gangue are formed, and

f. passing the fine particles of gangue to waste.

2. The improved continuous method of claim 1 in which the pulp density of the slurry in step (a) is between 3.0 and 3.6.

3. The improved continuous method of claim 1 in which the oversize fraction from step (c) is passed through a closed regrind circuit wherein the ground particles are concentrated to separate and recover liberated fine high grade mineral particles as a usable product from particles of gangue which are passed to waste.

4. The method of claim 1 wherein the overflow fraction of step (a) is classified in a hydrocyclone to separate and recover the fine high grade mineral particles as an overflow fraction from the coarse low grade middling particles as an underflow fraction, said underflow fraction being processedto recover mineral particles contained therein.

5. The method of claim 1 wherein the hydrocyclone overflow fraction of step (a) is classified on a screen to separate and recover fine high grade mineral particles as an underflow from coarse low grade middling particles as an overflow, said coarse low grade middling particles being further processed to recover mineral particles contained therein. 

2. The improved continuous method of claim 1 in which the pulp density of the slurry in step (a) is between 3.0 and 3.6.
 3. The improved continuous method of claim 1 in which the oversize fraction from step (c) is passed through a closed regrind circuit wherein the ground particles are concentrated to separate and recover liberated fine high grade mineral particles as a usable product from particles of gangue which are passed to waste.
 4. The method of claim 1 wherein the overflow fraction of step (a) is classified in a hydrocyclone to separate and recover the fine high grade mineral particles as an overflow fraction from the coarse low grade middling particles as an underflow fraction, said underflow fraction being processed to recover mineral particles contained therein.
 5. The method of claim 1 wherein the hydrocyclone overflow fraction of step (a) is classified on a screen to separate and recover fine high grade mineral particles as an underflow from coarse low grade middling particles as an overflow, said coarse low grade middling particles being further processed to recover mineral particles contained therein. 